1. Field of the Disclosure
Embodiments disclosed herein relate generally to a process for the oxidative dehydrogenation of hydrocarbons to form olefins. More specifically, embodiments disclosed herein relate to a process for the oxidative dehydrogenation of ethane to form ethylene. Such processes may be performed in the presence of a mixed metal oxide catalyst, allowing for exceptionally high selectivity to the olefin at all hydrocarbon conversion levels (from <20% to >90%).
2. Background
Ethylene is an important petrochemical used as a raw material for the manufacture of polymers, ethylbenzene, styrene, and polystyrene, among other chemical products. Over 90% of currently produced ethylene is derived from steam cracking of naphtha and/or ethane and/or propane. Ethylene may be obtained from the non-catalytic thermal cracking of saturated hydrocarbons, such as ethane and propane, and alternatively from thermal or steam cracking of heavier liquids such as naphtha and gas oil. Steam cracking produces a variety of other products, including diolefins and acetylene. The latter are costly to separate from the ethylene, usually by extractive distillation and/or selective hydrogenation to the corresponding mono-olefin, e.g. acetylene to ethylene. An ethylene plant using thermal cracking typically achieves an ethylene selectivity up to 80-85 percent calculated on a carbon atom basis at an ethane conversion of 55-65 percent. In addition, thermal cracking processes for olefin production are highly endothermic. Accordingly, these processes require a large consumption of fuel and the construction and maintenance of large, capital-intensive and complex cracking furnaces to supply the heat.
Existing steam cracking processes generate ethylene by raising the feed (ethane or other hydrocarbons) to high enough temperature (700-1000° C.) in furnace tubes to thermally crack the hydrocarbons into olefins, especially ethylene and secondarily propylene, plus a range of other hydrocarbons, hydrogen and coke. The residence time must be very short, at a level measured in milliseconds, and the effluent must be quenched immediately, in order to maximize the desired olefins and minimize the undesired by-products. The pressure must be kept to a minimum, substantial steam dilution is required, and design features are critical for obtaining the best performance. As a result, the reaction conditions are very sensitive, and the furnaces are very expensive, with high fuel requirement due to both the high temperature and the high endothermicity of the cracking reactions. Frequent decoking is also a major requirement. Furthermore, furnace tubes must be replaced periodically.
Autothermal cracking (“ATC”) is a similar process, but with a combustion reaction added to supply the heat, as an alternative to using expensive heat transfer in furnaces. The combustion reaction may include use of a catalyst, for which the high temperature and other conditions are a severe environment. There are still very sensitive cracking reactions and quenching, with a range of products, and the added combustion reactions create additional byproducts while consuming either a portion of the feed and product and/or a combustible that is added.
An alternative is to catalytically dehydrogenate ethane in the presence of oxygen to form ethylene. The process is called oxidative dehydrogenation (ODH). In this process, the product is largely limited to ethylene with small amounts of carbon monoxide and carbon dioxide as byproducts. The effluent also contains water (produced in the reaction plus whatever enters with the feed), residual ethane, some residual oxygen, and nitrogen if introduced with the oxygen (e.g., as air). The oxidative dehydrogenation (ODH) of ethane is thermodynamically favored and can be carried out at lower reaction temperatures without coke formation.
In U.S. Pat. No. 4,250,346, ethane is catalytically oxydehydrogenated to ethylene in a gas phase reaction, in the presence or absence of water, at temperatures of less than 550° C. The catalysts disclosed include oxides of molybdenum: MoaXbYc, where X=Cr, Mn, Nb, Ta, Ti, V and/or W, Y=Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, Tl and/or U.
U.S. Pat. No. 4,524,236 discloses catalysts useful for the production of ethylene from ethane via oxidative dehydrogenation, including oxides of molybdenum: MoaVbNbcSbdXc, where X=Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, Te, U, and W. The reaction can be carried out in the presence or absence of water; however, significant amounts of acetic acid are formed in the presence of water, which results in reduced ethylene selectivity.
U.S. Pat. No. 6,858,768 discloses catalysts useful for the production of olefins from alkanes via oxidative dehydrogenation, including an oxide selected from the group containing alumina, zirconia, titania, ytria, silica, niobia, and vanadia. As disclosed, the catalysts need substantially elevated temperatures for activation.
U.S. Pat. No. 7,319,179 discloses mixed metal oxide catalysts comprising molybdenum, vanadium, tellurium, and niobium useful as a catalyst for ODH of ethane to ethylene.
JP 07-053414 discloses use of mixed metal oxide catalysts containing transition metal elements with molybdenum, vanadium, niobium, and tellurium for the ODH of ethane to ethylene. The best selectivity reported therein is 91.5 C % ethylene at 56.7% conversion at a reaction temperature of 360° C.
Other patents discussing ODH of ethane to ethylene include U.S. Pat. Nos. 6,858,768, 7,135,603, 4,940,826, 6,433,234, and 6,566,573. Various other references discussing ODH include: P. Botella, E. Garcia-Gonzalez, A. Dejoz, J. M. Lopez-Nieto, M. I. Vazquez, and J. Gonzalez-Calbet, “Selective oxidative dehydrogenation of ethane on MoVTeNbO mixed metal oxide catalysts,” Journal of Catalysis 225: 428-438, 2004; Q. Xie, L. Chen, W. Weng, and H. Wan “Preparation of MoVTe(Sb)Nb mixed oxide catalysts using a slurry method for selective oxidative dehydrogenation of ethane,” Journal of Molecular Catalysis A. 240: 191-196, 2005; and Grabowski, R. “Kinetics of oxidative dehydrogenation of C2-C3 alkanes on oxide catalysts,” Catal. Rev. Sci and Eng'g. 48: 199-268, 2006.
Due to the potential advantages over the prior art, ODH of ethane to ethylene has been the object of considerable research. Over the years, many catalyst systems have been investigated, including carbon molecular sieves, metal phosphates, and mixed metal oxides. However, commercialization has not been possible due to low product selectivity at reasonably high ethane conversions. In many of the prior art processes using ODH to form ethylene, the oxygen has generated excessive byproducts (primarily COx), with selectivity to the desired ethylene product reaching no higher than 80-85 C % at ethane conversion of 55-65%. At this level of selectivity and conversion, no advantage over steam cracking is realized, especially as the primary by-products (COx) do not provide added value, in contrast to significant value for the hydrocarbon byproducts from steam cracking.
Accordingly, there remains a need in the art for ODH processes having high selectivity at reasonably high hydrocarbon conversions.